Method and apparatus for enhancing signal-to-noise ratio of position location measurements

ABSTRACT

A method and apparatus for generating and use of a position location reference signal that allows a receiver to receive position location signals from relatively weak signal generators when in the presence of a strong signal source. The position location reference signals from multiple sources can be synchronized to occur within a scheduled time slot of a time division multiplexed communication system. During the scheduled time slot, each signal source can configure a transmission that includes a media access control address that corresponds to a value reserved for position location signals. Each signal source also configures the transmission to include a position location reference signal that corresponds to the signal source. The position location signals from each of the signal sources is positioned to occur at a time within the data portion of the scheduled time slot that no neighboring signal source transmits its corresponding position location signal.

This application is a divisional of application Ser. No. 11/120,411,which was filed on May 2, 2005, is herein incorporated by reference, andclaims priority from U.S. Provisional Application No. 60/666,138, whichwas filed on Mar. 28, 2005, and is also herein incorporated byreference.

This disclosure relates generally to electronic communications. Moreparticularly, the disclosure relates to wireless position location, andsignals in a wireless position location system.

BACKGROUND OF THE DISCLOSURE

In many applications it may be advantageous to have the ability todetermine a position of a mobile device. Position location may behelpful for navigation, tracking, or orientation applications. Thecontinual advancement of the performance of portable electronics,particularly the advancements in the performance of processors, allowsposition location capabilities to be added in a variety of devices.

For example, it may be desirable for an operator of a mobiletelecommunications system such as a cellular telecommunications systemto be able to determine the position of a mobile handset duringcommunication with a base transceiver station (BTS) of the system. Asystem operator may desire position location capabilities, for example,to satisfy the U.S. Federal Communications Commission (FCC) E911emergency position location mandate.

Mobile devices may implement one or more position location techniquesdepending on the position location signaling methods used in theposition location system. For example, a mobile device may use time ofarrival (TOA), time difference of arrival (TDOA), advanced forward linktrilateration (AFLT) or some other position location technique. Examplesof position location systems include those that are based on the GlobalPositioning System (GPS), those that augment (he GPS system withterrestrial based beacons such as hybrid position location systems, andterrestrial based beacon position location systems. In one example, themobile device can determine its position by determining absolute delaymeasurements to two terrestrial beacons or relative delay measurementsto at least three terrestrial beacons.

Most terrestrial ranging systems incorporate a pseudo noise (PN) code ina direct sequence spread spectrum configuration. Each position locationbeacon can transmit a PN code that identifies the beacon. In a timedivision multiplexed forward link communication system, the positionlocation beacons can be synchronized to transmit a correspondingposition location PN code at substantially the same time.

A mobile device can identify a particular source, in part, bycorrelating a received PN spread signal with an internally generatedversion. However,; a mobile position location device in a terrestrialbased system can receive widely disparate signal powers. One problemthat mobile devices encounter is associated with the receiving widelydisparate signal powers.

In certain situations, the mobile device is only able to determine theposition location signal from one beacon because the signal from thebeacon is so strong that it interferes With signals from other beacons.The signal to noise ratio of the position location signals from thesurrounding beacons is too low for the mobile device to extract usefulranging measurements. A mobile device may experience this effect when itis relatively near one beacon and relatively far from surroundingbeacons. The effect is typically referred to as the near-far problem.

Therefore, it is desirable to have a position location signalingtechnique, system, and device that allow for high performance positionlocation in a variety of conditions, including a near-far condition, andyet may be implemented in a practical manner.

BRIEF SUMMARY OF THE DISCLOSURE

A method is disclosed for determining a position of a mobile device in atime division multiplexed communication system, the method comprising:receiving a synchronized position location slot, wherein thesynchronized position location slot includes a plurality of sub-slotsassigned to a plurality of position location signal sources, and anidentifier indicating the synchronized position location slot includesposition location information; determining from a first sub-slot in thesynchronized position location slot, a first position location signalcorresponding to a first position location signal source; determiningfrom a second sub-slot in the synchronized position location slot, asecond position location signal corresponding to a second positionlocation signal source; and determining a timing related to positionbased on the first and the second position location signals.

An apparatus is disclosed for determining a position of a mobile devicein a time division multiplexed communication system, comprising: meansfor receiving a synchronized position location slot, wherein thesynchronized position location slot includes a plurality of sub-slotsassigned to a plurality of position location signal sources, and anidentifier indicating the synchronized position location slot includesposition location information; means for determining, from a firstsub-slot in the synchronized position location slot, a first positionlocation signal corresponding to, a first position location signalsource; means for determining, from a second sub-slot in thesynchronized position location slot, a second position location signalcorresponding to a second position location signal source; and means fordetermining a timing related to position based on the first and thesecond position location signals.

A computer-readable medium including program code, stored thereon, isdisclosed for determining a position of a mobile device in a timedivision multiplexed communication, system, comprising: program code toreceive a synchronized position location slot, wherein the synchronizedposition location slot includes a plurality of sub-slots assigned to aplurality of position location signal sources, and an identifierindicating the synchronized position location slot includes positionlocation information; program code to determine from a first sub-slot inthe synchronized position location slot, a first position locationsignal corresponding to a first position location signal source; programcode to determine from a second sub-slot in the synchronized positionlocation slot, a second position location signal corresponding to asecond position location signal source; and program code to determine atiming related to position based on the first and the second positionlocation signals.

An apparatus is disclosed for determining a position of a mobile devicein a time division multiplexed communication system, comprising: an RFfrontend for receiving a synchronized position location slot, whereinthe synchronized position location slot includes a plurality ofsub-slots assigned to a plurality of position location signal sources,and an identifier indicating the synchronized position location slotincludes position location information; and a baseband processor for:determining from a first sub-slot in the synchronized position locationslot, a first position location signal corresponding to a first positionlocation signal source; determining from a second sub-slot in thesynchronized position location slot, a second position location signalcorresponding to a second position location signal source; anddetermining a timing related to position based on the first and thesecond position location signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of embodiments of the disclosurewill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings, in which like elements bearlike reference numerals.

FIG. 1 is a simplified functional block diagram of an embodiment of aposition location system.

FIGS. 2A-2B are block diagrams of embodiments of slot data structures.

FIG. 3 is a simplified functional block diagram of an embodiment of aposition location signal source.

FIG. 4 is a simplified functional block diagram of an embodiment of auser terminal having position location capabilities.

FIG. 5 is a simplified flowchart of an embodiment of a method ofgenerating a position location data slot.

FIG. 6 is a simplified flowchart of an embodiment of a method of using aposition location data slot.

DETAILED DESCRIPTION OF THE DISCLOSURE

A position location system that includes terrestrial-based beacons canvirtually eliminate the effects of the near-far problem by implementinga position location signaling structure that allows a receiver toreceive and measure the position location signals in a relatively noisefree environment. To minimize the noise and interference contributed bysurrounding beacons, each beacon can be configured to transmit itscorresponding position location signal at a time that no surroundingbeacon transmits a position location signal or a data signal.

In an asynchronously scheduled time division multiplex forward linkcommunication system, a time slot can be scheduled as a positionlocation time slot. Each signal source can configure the time slot inthe manner specified for a typical data slot. However, the positionlocation time slot'can be divided into a plurality of sub-slots that arepositioned within what is normally the data field of a data slot. Eachsignal source can be assigned to a corresponding one of the sub-slotsand can transmit its corresponding position location signal during theassigned sub-slot. A packet identifier, such as a MAC address includedwithin the position location time slot, can identify the informationcontained within the time slot as position location information. Theposition location time slot can be periodically scheduled or can bescheduled in response to a request for position location information.

FIG. 1 is a simplified functional block diagram of an embodiment of aposition location system 100 that includes terrestrial beacons. Theposition location system 100 can include one or more terrestrialelements that can be in communication with a user terminal 110. The userterminal 110 can be, for example, a wireless telephone configured tooperate according to one or more communication standards. The one ormore communication standards can include, for example, GSM, WCDMA, andCDMA2000 including 1x EV-DO, 1X EV-DV, and CDMA2000 3x. The userterminal 110 can be a portable unit, a mobile unit, a stationary unit.The user terminal 110 may also be referred to as a mobile unit, a mobileterminal, a mobile station, user equipment, a portable, a phone, and thelike.

The user terminal 110 typically communicates with one or more basestations 120 a or 120 b, here: depicted as sectored cellular towers. Theuser terminal 110 will typically communicate with the base station, forexample 120 b, that provides the strongest signal strength at a receiverwithin the user terminal 110. Two base stations 120 a and 120 b and oneuser terminal 110 are shown in FIG. 1 for the sake of convenience andclarity. A system typically has numerous base stations and can supportmore than one user terminal.

The user terminal 110 can determine its position, in part, based onposition location signals received from one or more signal sources. Thesignal sources can include one or more satellites 130, that can be partof a satellite based position location system, such as the GlobalPositioning System (GPS). The signal sources can also include the one ormore base stations 120 a or 120 b.

Each of the base stations 120 a and 120 b can be coupled to a BaseStation Controller (BSC) 140 that routes the communication signals toand from the appropriate base stations 120 a and 120 b. The BSC 140 canbe coupled to a Mobile Switching Center (MSC) 150 that can be configuredto operate as an interface between the user terminal 110 and a PublicSwitched Telephone Network (PSTN) 170. Therefore, the MSC 150 is alsocoupled to the PSTN 170. The MSC 150 can also be configured tocoordinate inter-system handoffs with other communication systems.

A Position Location Center (PLC) 160 can also be coupled to the BSC 140.The PLC 160 can be configured, for example, to store position locationinformation, such as the location of each of the base stations 120 a and120 b in the position location system 100. In one embodiment, the PLC160 can be configured to provide the information to the user terminal110 such that the user terminal 110 can determine its location based inpart on pseudo ranges to multiple signal sources, where the pseudoranges can be relative time-of-arrival values. In another embodiment,the PLC 160 can be configured to determine the location of the userterminal 110 based on pseudo range information provided by the userterminal 110. In the latter embodiment, a network server (not shown) inthe PLC 160 can perform the position location determination in order tooffload processing from the user terminal 110.

The PLC 160 can be configured to command the base stations 120 a and 120b, via the BSC 140, to generate the position location signals. In otherembodiments, the base stations 120 a and 120 b can be configured toperiodically generate the position location signals.

The base stations 120 a and 120 b in a time division multiplex forwardlink system can be configured to generate the position location signalsin a manner that allows the user terminal 110 to receive a positionlocation signal with relatively high SNR from a remote base station, forexample 120 a, even when in the presence of a strong position locationsignal source, for example 120 b.

The base stations 120 a and 120 b can be configured to be synchronizedto a common system time and can be configured to transmit a positionlocation signal within a predetermined time slot. Each base station, forexample 120 b, can be configured to transmit within the time slot anidentifier that can be used to identify the base station from which thesignal originated. The identifier can be, for example, a pilot signalburst occurring within a predetermined time during the time slot.

Each time slot can also be configured to include a media access control(MAC) field that includes the MAC address of a destination device. Forexample, if a base station 120 b is transmitting a data packet that isintended for the user terminal 110, the time slot includes a MAC addressof the user terminal 110. The user terminal 110 can determine if it isthe intended recipient of a received packet by examining the MACaddress. The user terminal 110 can ordinarily ignore data packetsaccompanying time slots that do not have the MAC address correspondingto the MAC address of the user terminal 110.

A position location time slot can include a MAC address that is reservedfor position location data. The base stations 120 a and 120 b use thereserved MAC address when transmitting the position location signals.The user terminal 110 can be configured to examine received packets forthe reserved MAC address when attempting position location.

The position location time slot can be further divided into multiplesub-slots, and each base station 120 a and 120 b can be assigned to atleast one of the sub-slots in at least on e of the position locationtime slots. The base stations 120 a and 120 b within the positionlocation system 100 can be assigned sub-slots in accordance with a reusescheme that ensures that the same sub-slot is not assigned to basestations having overlapping coverage areas. In one embodiment, each basestation 120 a and 120 b is assigned to a particular sub-slot in theposition location time slot. In another embodiment, each sector of abase station 120 a and 120 b can be assigned to a distinct sub-slotwithin the position location time slot.

Each base station, for example 120 a, can be configured to transmit acorresponding position location signal during its assigned sub-slot. Thebase stations 120 a and 120 b do not transmit during the sub-slot forwhich they are not assigned. The user terminal 110 then effectivelyreceives position location signals from one base station 120 a and 120 bor base station sector during any sub-slot of the position location timeslot. Because only a single base station 120 a and 120 b or base stationsector transmits during any particular sub-slot, the user terminal 110is able to recover the position location signal in a substantially noisefree environment, where noise free refers to the noise and interferencegenerated by surrounding base stations.

FIG. 2A is a block diagrams of an embodiment of an existing slot datastructure 200 in a time division multiplex forward link communicationsystem. The time slot structure 200 can be used, for example, by thecommunication system shown in FIG. 1. The slot data structure 200 cancorrespond to a time slot occurring in a CDMA 2000 1xEV-DO communicationsystem. The existing slot data structure 200 is used to illustrate howthe position location signaling can be configured to be backwardcompatible with legacy devices operating in a communication system. Theposition location methods and apparatus are not limited toimplementation in a CDMA2000 1xEV-DO system, but may be implemented invirtually any type of time division multiplex forward link system.

The existing slot data structure 200 includes a time slot that isdivided into two half slots 210 a and 210 b. In one embodiment, eachhalf slot 210 a and 210 b can be configured independently of the other.In another embodiment, the half slots 210 a and 210 b can contain thesame information.

Each half slot, for example 210 a, includes a pilot burst field 220 athat is used by the transmitting base station to transmit the pilotchannel corresponding to the base station. A user terminal can use thepilot bursts, for example, to initially synchronize with the basestation and to determine the identity of the base station.

Each half slot, for example 210 a, includes MAC fields occurring before222 a and after 224 a the pilot burst field 220 a. The MAC fields 222 aand 224 a can be used to identify the recipient of the data in the halfslot 220 a. In one embodiment, the first MAC field 222 a and second MACfield 224 a contain the same MAC address. In another embodiment, thefirst and second MAC fields 222 a and 224 a can contain differentaddresses. In another embodiment, the MAC address can be contained in acombination of the first MAC field 222 a and second MAC field 224 a.

Within each half slot, for example 210 a, there exists data portions 232a and 234 a. The data portions 232 a and 234 a can be used tocommunicate data to a destination device identified by the correspondingMAC address, such as a user terminal.

FIG. 2B is a block diagrams of an embodiment of a position location slotdata structure 250 in a time division multiples communication systemconfigured for position location information. The position location slotdata structure 250 is illustrated from the perspective of the receiver,such as, the user terminal. The differences between the receiver andtransmitter perspective will be discussed below.

The position location slot data structure 250 includes two half slots210 a and 210 b. In the block diagram of FIG. 2B, the two half slots 210a and 210 b are shown separately for the purposes of clarity. Thediscussion will focus on the first half slot 210. However, the structureof the second half slot 210 b can be similar to that of the first halfslot 210 a.

The first half slot 210 a includes a pilot burst field 220 a and firstand second MAC fields 226 a and 228 a. The timing of the pilot burstfield 220 a and first and second MAC fields 226 a and 228 a can be thesame as was used in the existing slot data structure 200 discussed abovein relation to FIG. 2A.

In a position location half slot 210 a, the MAC address does notcorrespond to the MAC address of the recipient user terminal. Instead,each position location signal source, such as a base station, inserts areserved MAC address into the first and second MAC fields 226 a and 228a. The reserved MAC address corresponds to a MAC address that does notcorrespond to any particular receiver in the coverage area supported bythe base stations, and may not correspond to any MAC address that can beassigned to any user terminal in the entire system.

In one embodiment, the reserved MAC address is a static MAC address thatis configured for the entire system. Each user terminal, or receiver ingeneral, that is configured to process the position location signals canbe configured to monitor for the reserved MAC address. In anotherembodiment, the MAC address can be dynamic, and can be assigned inresponse to a request for position location signals. The base stationsin a location servicing the requesting user terminal can be assigned aMAC address that does not correspond to the MAC address of any userterminal within its coverage areas. The reserved MAC address can becommunicated to the user terminal, for example, using an overheadchannel, and the user terminal can monitor for the assigned MAC address.

The data portions of half slots 210 a and 210 b of a position locationslot can be further divided into sub-slots. Each of the sub-slots can beassigned to a base station, and the base station can transmit itsposition location signal in the assigned sub-slot.

The user terminal monitors the slot for the reserved MAC address andprocesses the entire received slot based on the MAC address. However,the slot includes multiple sub-slot information transmitted by multiplebase stations. Each base station only transmits its position locationsignal in its assigned sub-slot, and the receiver in the user terminalreceives the aggregate signal, which can include multiple sub-slottransmissions corresponding to multiple base stations. The positionlocation slot data structure 250 of FIG. 2B shows five separatesub-slots within each half slot 210 a and 210 b. However, the positionlocation slot data structure 250 is not limited to having fivesub-slots, but may implement any number of sub-slots based on theduration of the slot and the duration of the sub-slots and guardperiods.

The sub-slots can be configured to be immediately adjacent to oneanother or an be configured to have some time between adjacentsub-slots. For example, a first sub-slot 260 a can be configured tooccur at the leading edge of the first half slot 210 a. A secondsub-slot 262 a can be configured to occur after the first sub-slot 260a. There can be a guard period 270 a between the first sub-slot 260 aand the second sub slot 262 a.

The guard period 270 a can be used to minimize the possibility that afirst position location signal transmitted by a first base stationassigned to the first sub-slot 260 a will overlap a second positionlocation signal transmitted by a second base station assigned to thesecond sub-slot 262 a.

A timing overlap of adjacent position location signals can occur due todifferences in propagation delays that are typically attributable to thedistance of the receiver to the signal source. Therefore, if the userterminal is physically close to the second base station and relativelydistant from the first base station, the position location signal fromthe first base station assigned, for example, to the first sub-slot 260a will be delayed relative to the position location signal transmittedby the second base station in the second sub-slot 262 a. If there is noguard period 270 a, the user terminal may experience some overlap of thetwo position location signals.

The duration of the guard period 270 a can be predetermined and can bebased on the farthest distance between the two base stations for which asingle user terminal may expect to receive position location signals.Alternatively, the duration of the guard period 270 a can be determinedbased on a nominal base station distance and an acceptable overlapduration.

In other embodiments, the guard period 270 a can be eliminated and thesub-slots can be positioned immediately adjacent one another. Theembodiment where no guard period exists between adjacent sub-slots isshown in the sub-slots occurring after the pilot burst 220 a and MACfields 226 a and 228 a of the first half slot 210 a.

A third sub-slot 264 a occurs immediately following the second MAC field228 a. A fourth sub-slot 266 a immediately follows the third sub-slot264 a. Similarly, a fifth sub-slot 268 a immediately follows the fourthsub-slot 266 a.

The guard period 270 a can be eliminated, for example, where the overlapof adjacent sub-slot information is envisioned to be insufficient tocause degradation of the position location signals. The degradation canbe insufficient to cause degradation because of the duration of theoverlap is minimal, because the position location signaling isinsensitive to timing overlap, or because of a combination of factors.

The position location signals transmitted by each base station can beany one of various position location signals. As described earlier, theposition location signals can be PN codes or other codes with preferablecorrelation properties, and each base station can be assigned one of aplurality of PN codes or PN code offsets.

In one embodiment, the position location signals transmitted by eachbase station corresponds to the pilot burst for that base station. Inother embodiments, the position location signals can be chosen from PNcodes that have favorable cross correlation properties. That is, a PNcode assigned to a first base station will have low cross correlationwith a PN code assigned to a second base station. The use of positionlocation signals having low cross correlation can be used to minimize oreliminate the need for guard periods 270 a.

A base station thus transmits a position location signal in its assignedsub-slot when no other base station in the surrounding area istransmitting. To the user terminal, the composite position location slotappears as multiple position location signals transmitted in acorresponding multiple of sub-slots. The information in each sub-slotcan be received in a substantially noise free environment because onlyone base station is transmitting during each sub-slot.

FIG. 3 is a simplified functional block diagram of an embodiment of aposition location signal source 300. The position location signal source300 can include, for example, a base station of the system of FIG. 1, orcan be a portion of the base station in combination with some or all ofthe BSC 140 and PLC 160.

The position location signal source 300 includes a RF transceiver 310coupled to an antenna 302. The RF transceiver 310 can be configured toreceive ranging signals from the user terminals (not shown) and couplethem to a PLC 160 for position determination. In one embodiment, the RFtransceiver 310 can also receive a request for transmission of aposition location time slot. In such an embodiment, the positionlocation signal source 300 can be configured to transmit positionlocation time slots in response to requests from one or more userterminals. In another embodiment, the position location signal source300 can be configured to periodically transmit position location timeslots.

The RF transceiver 310 can couple received ranging requests to a rangingrequest decoder 320. Alternatively, the PLC 160 or some other module(not shown) can generate a signal to the ranging request module 320 toschedule transmission of a position location signal.

The ranging request module 320 can configure or otherwise command aranging data module 330 to generate a ranging data, which can be theposition location signal that the RF transceiver 310 transmits duringthe assigned sub-slot. In one embodiment, the position location signalcorresponds to the pilot signal for the base station. In otherembodiments, the position location signal can be a PN code or some otherposition location signal. The ranging data module 330 can also beconfigured to supply the reserved MAC address that is used in theposition location timing slots.

The ranging data module 330 can couple the position location signal andreserved MAC address to a slot processor 340 that configures the slotsfor transmission. The slot processor 340 can be configured tosynchronize the timing of the slots with a system time reference.

The slot processor 340 can also be configured to receive the data thatis to be transmitted to the various user terminals within the range ofthe RF transceiver 310. The slot processor 340 schedules the positionlocation slot and configures the position location slot to have thereserved MAC address and pilot burst for the corresponding base station.The slot processor 340 can also be configured to configure the positionlocation signal to the assigned sub-slot. The slot processor 340 couplesthe configured slots to the RF transceiver 310 to transmit to the userterminals, via the antenna 302.

FIG. 4 is a simplified functional block diagram of an embodiment of auser terminal 110 having position location capabilities. The userterminal 110 can be, for example, the user terminal of the system shownin FIG. 1.

The user terminal 110 includes an antenna 402 coupled to an RF frontend410. The RF frontend 410 can be a transceiver that is configured todownconvert and process received signals as well as upconvert andprocess baseband signals for transmission. The RF frontend 410 canprocess all received signals within an operating band. These signals mayinclude data intended for the user terminal, data intended for otheruser terminals, overhead information, as well as position locationinformation.

An analog to digital converter (A/D) 422 can be coupled to a receivesignal output from the RF frontend 410. The A/D 422 can couple thedigitized signal to an input of a baseband processor 430.

The output of the A/D 422 can be coupled to a decoder 440 within, thebaseband processor 430. The decoder 440 can be configured, for example,to decode encoded symbols that are received and recover thecorresponding bits. The decoder 440 may also perform demodulation,dispreading, or some other signal processing of the received signal,depending on the implementation in the system.

The output of the decoder 440 can be coupled to a buffer 442 and a MACdecoder 444. If the slot data structure is similar to that shown in,FIG. 2A-2B, the MAC address occurs in MAC fields that occur after a datafield. The user terminal 110 buffers the slot information in the buffer442 until the MAC decoder 444 is able to decodes the MAC address in theslot.

If the MAC address of the received slot does not correspond to the MACaddress of the user terminal 110 or if the received MAC address does notcorrespond to the reserved MAC for position location slots, the basebandprocessor 430 can determine that the information in the slot is notintended for the user terminal 110 and can discard or otherwise ignorethe information in the buffer 442 and remaining informationcorresponding to the slot.

If the MAC decoder 444 determines that the MAC address for the receivedslot corresponds to the MAC address for the user terminal 110, the MACaddress can direct the received signals to a signal processor (notshown) that can process the received data. If the MAC decoder 444determines that the MAC address for the received slot corresponds to thereserved MAC for position location slots, the MAC decoder can enable orotherwise direct the received signals to a position location timingmodule 450. Therefore, when the MAC address indicates receipt of aposition location slot, the received data is coupled from the buffer 442to the position location timing module 450.

The position location timing module 450 can determine the position ofthe user terminal 110 based in part on the position location signalsreceived in the position location timing slot if the user terminal 110is configured for mobile station based position location. In manysystems, the user terminal is not configured for mobile station basedposition location; instead, the user terminal determines one or morepseudo ranges corresponding to the received position location signalsand transmits the pseudo ranges to a network or, for example, to a PLCin a position location system, where the location of the user terminal110 is determined. The latter method can be referred to as mobilestation assisted position location.

If the user terminal 110 is configured for mobile station assistedposition determination, the position location timing module candetermine one or more pseudo ranges corresponding to the signalsreceived in the position location slot and can couple the pseudo rangevalues to a timing module 470 that is configured to configured the oneor more pseudo range values for transmission to a network for positiondetermination.

The timing module 470 couples the information to a transmit signalprocessor 480 that can be configured to encode and modulate theinformation in a manner specified for the system. The transmit signalprocessor 480 can couple the processed signal to a digital to analogconverter (D/A) 424 where the signal is converted to an analogrepresentation.

The analog representation is coupled from the D/A 424 to the transmitpath of the RF frontend 410 where it can be processed into a transmitsignal. The RF frontend 410 couples the transmit signal to the antenna402 where it is transmitted to its destination, which can be, forexample, a base station.

As described above, in one embodiment the user terminal 110 can requesta position location slot. The position location timing module 450 can beconfigured to initiate or command a position location request generator460 when position determination is desired.

The position location request generator 460 can generate a request for aposition location slot. The request can be a simple request thatidentifies the user terminal 110, or can be a more detailed request thatincludes such information as the identities of the base stations on acandidate list or neighbor list maintained by the user terminal 110. Thecandidate list or neighbor list can be maintained by the user terminal110 as part of operating in the communication system. For example, theuser terminal 110 can maintain a candidate list and a neighbor list tomonitor the signal powers of base stations for potential handoffs.

FIG. 5 is a flowchart of an embodiment of a method 500 of generating aposition location slot in a time division multiplex forward link system.The method 500 can be performed by the base stations of the system ofFIG. 1 or the signal source of FIG. 3.

The base station can initiate the method 500 for each of a periodicposition location slot or can initiate the method 500 in response to arequest for a position location slot. The method begins at block 520where the base station synchronizes its slot timing with a system time.In embodiments where the base station is part of a larger time divisionmultiplex forward link communication system, the base station mayalready be synchronized to a system time regardless of any positionlocation signaling.

After synchronizing the slot timing with a system time, the base stationcan proceed to block 520 where the base station determines the timing ofthe position location slot. The scheduling of the position location slotwill be coordinated across multiple base stations, such that all of thebase stations will transmit the position location signals in apredetermined time within the time slot. The base station may receivethe scheduling or timing of the position location slot from, forexample, a slot processor that can be implemented in a BSC.

After determining the timing of the position location slot, the basestation can proceed to block 530 to configure the MAC address of theposition location slot. The base station can configure the MAC addressto be a reserved MAC address that is used for position location slots,and is not assigned to any user terminal. Alternatively, a slotprocessor or ranging data module can be provide a MAC address for thebase station to use in the position location slot.

The base station proceeds to block 540 and configures the positionlocation slot by configuring the position location signal in thesub-slot assigned to the base station. The position location signalcorresponding to the base station can be, for example, a pilot signalthat the base station transmits or can be come other signal such as a PNcode, Walsh code, Gold code, or some other sequence.

The base station proceeds to block 550 and blanks the remainingsub-slots of the position location slot. The base station can beconfigured to actively blank the sub-slots for which it is not assigned.Alternatively, the base station can configure the sub-slots for which itis not assigned to have no data.

After configuring the complete position location slot, the base stationproceeds to block 560 and transmits the position location slot at thepreviously determined time. The base station can repeat the method 500periodically or can repeat the method 500 based on a position locationrequest.

FIG. 6 is a simplified flowchart of an embodiment of a method 600 ofusing a position location data slot. The method 600 can be performedwithin a user terminal, such as the user terminal of FIGS. 1 and 4.

The method 600 begins at block 610 when the user terminal receives, adata slot. The position location slot can be configured to occur in atime slot that is configured similar to a typical data slot.

The user terminal proceeds to block 612 and buffers the received data.If the data structure of the position location slot is similar to thatshown in FIG. 2B, the user terminal cannot determine the MAC addressassociated with the data slot until after a portion of the data has beenreceived. Thus, the user terminal can buffer the received data at leastuntil the user terminal can determine the slot MAC address.

The user terminal proceeds to block 620 and determines the MAC addressassociated with the data slot. The user terminal can determine the MACaddress in the same manner that is performed for other data slots.

The user terminal proceeds to decision block 630 and determines whetherthe MAC address determined in the previous block corresponds to the MACaddress of the user terminal. If so, the user terminal proceeds to block632 to process the received data in a conventional manner specified bythe user terminal. After processing the data in the conventional manner,the user terminal can proceed to block 680 and the method 600 is done.

Returning to decision block 630, if the user terminal determines thatthe MAC address of the received slot does not correspond to the MACaddress of the user terminal, the user terminal proceeds to decisionblock 640 to determine whether the received MAC corresponds to the MACthat is reserved for position location slots.

If not, the MAC address probably corresponds to the MAC of some otherdevice and the data in the received slot is not intended for the userterminal. The user terminal proceeds to block 680 and the method 600 isdone.

Returning to decision block 640, if the user terminal determines thatthe MAC from the received slot corresponds to the reserved positionlocation MAC, the contents of the slot correspond to position locationsignals. The user terminal proceeds to block 650 to process the positionlocation signals.

In block 650, the user terminal determines the timing for each of theposition location signals included in the slot. In one embodiment, theuser terminal can be synchronized to a system time and can determine atime offset or delay corresponding to the position location signals ineach of the sub-slots. Because the signals in each of the sub-slots cancorrespond to a different signal source, the user terminal can determinea time offset corresponding to each of the signal sources. The userterminal can determine an equivalent pseudo range corresponding to thetiming offset, or can determine some other result using the timingoffset.

The user terminal proceeds to block 660 and communicates the timing orvalues determined from the timing to a position location module. Theposition location module can be within the user terminal for mobilestation based position location, or can be in a network for network ormobile station assisted position location. In the latter case, where theposition determination is performed in a centralized location, the userterminal can transmit the timing information to a base station to beforwarded to a PLC.

The user terminal proceeds to block 670 and determines its location. Ifthe user terminal is configured for local position determination, theuser terminal can determine its location without further assistance. Ifthe user terminal is configured for mobile station assisted positiondetermination, such as where position determination is performed in acommon PLC coupled to the base station, the user terminal can determineits position in a message delivered from the PLC to the user terminal.In other embodiments, the user terminal may not determine its locationand the block can be omitted. For example, the position determinationmay only be accessible external to the user terminal, such as when theposition determination is used by emergency personnel in response to anemergency call.

Methods and apparatus are disclosed for improving the SNR of a positionlocation signal and for generating position location signals in a mannerthat is compatible with legacy devices. The SNR of the position locationsignals can be improved in a time division multiplex forward link systemby configuring a data slot to convey position location information. Theposition location information can be concurrently broadcast by multiplesignal sources, each transmitting a portion of an aggregate positionlocation slot received by the user terminal.

The position location time slot can be divided into a plurality ofsub-slots. Each base station, or sector of a base station, can beassigned to a particular sub-slot according to a sub-slot reusealgorithm. Each signal source then transmits a position location signalwithin the assigned sub-slot and does not transmit during thosesub-slots for which it is not assigned.

A user terminal can receive an aggregate or composite position locationslot that includes the position location signals transmitted by multiplesignal sources, each corresponding to a sub-slot within the positionlocation slot. The receiver can receive position location signals withhigh SNR because each signal source transmits its corresponding positionlocation signal during a sub-slot that is relatively noise free, as noother signal source is intentionally transmitting a signal during anon-assigned sub-slot.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), a Reduced Instruction Set Computer (RISC) processor, anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. A generalpurpose processor may be a microprocessor, but in the alternative, theprocessor may be any processor, controller, microcontroller, or statemachine. A processor may also be implemented as a combination ofcomputing devices, for example, a combination of a DSP and amicroprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration.

A software module may reside in RAM memory, flash memory, non-volatilememory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, aremovable disk, a CD-ROM, or any other form of storage medium known inthe art. An exemplary storage medium is coupled to the processor suchthe processor can read information from, and write information to, thestorage, medium. In the alternative, the storage medium may be integralto the processor.

The steps of a method, process, or algorithm described in connectionwith the embodiments disclosed herein may be embodied directly inhardware, in a software module executed by a processor, or in acombination of the two. The various steps or acts in a method or processmay be performed in the order shown, or may be performed in anotherorder. Additionally, one or more process or method steps may be omittedor one or more process or method steps may be added to the methods andprocesses. An additional step, block, or action may be added in thebeginning, end, or intervening existing elements of the methods andprocesses.

The above description of the disclosed embodiments is provided to enableany person of ordinary skill in the art to make or use the disclosure.Various modifications to these embodiments will be readily apparent tothose of ordinary skill in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the disclosure. Thus, the disclosure is not intendedto be limited to the embodiments shown herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

1. A method of determining a position of a mobile device in a timedivision multiplexed communication system, the method comprising:receiving a synchronized position location slot, wherein thesynchronized position location slot includes a plurality of sub-slotsassigned to a plurality of position location signal sources, and anidentifier indicating the synchronized position location slot includesposition location information; determining from a first sub-slot in thesynchronized position location slot, a first position location signalcorresponding to a first position location signal source; determiningfrom a second sub-slot in the synchronized position location slot, asecond position location signal corresponding to a second positionlocation signal source; and determining a timing related to positionbased on the first and the second position location signals.
 2. Themethod of claim 1, wherein the identifier comprises a MAC addressreserved for position location.
 3. The method of claim 1, wherein thefirst position location signal comprises a pilot burst from the firstposition location signal source.
 4. The method of claim 1, wherein thefirst position location signal comprises a PN sequence corresponding tothe first position location signal source.
 5. The method of claim 1,wherein the first sub-slot is separated in time from the second sub-slotby a predetermined guard period.
 6. The method of claim 1, wherein thesecond sub-slot immediately follows the first sub-slot.
 7. The method ofclaim 1, wherein the second position location signal is received duringa period of time in which the first position location signal sourcetransmits no symbols.
 8. The method of claim 1, further comprisingtransmitting the timing related to position based on the first and thesecond position location signals to a position location center.
 9. Anapparatus for determining a position of a mobile device in a timedivision multiplexed communication system, comprising: means forreceiving a synchronized position location slot, wherein thesynchronized position location slot includes a plurality of sub-slotsassigned to a plurality of position location signal sources, and anidentifier indicating the synchronized position location slot includesposition location information; means for determining, from a firstsub-slot in the synchronized position location slot, a first positionlocation signal corresponding to a first position location signalsource; means for determining, from a second sub-slot in thesynchronized position location slot, a second position location signalcorresponding to a second position location signal source; and means fordetermining a timing related to position based on the first and thesecond position location signals.
 10. The apparatus of claim 9, whereinthe identifier comprises a MAC address reserved for position location.11. The apparatus of claim 9, wherein the first position location signalcomprises a pilot burst from the first position location signal source.12. The apparatus of claim 9, wherein the first position location signalcomprises a PN sequence corresponding to the first position locationsignal source.
 13. The apparatus of claim 9, wherein the first sub-slotis separated in time from the second sub-slot by a predetermined guardperiod.
 14. The apparatus of claim 9, wherein the second sub-slotimmediately follows the first sub-slot.
 15. The apparatus of claim 9,wherein the second position location signal is received during a periodof time in which the first position location signal source transmits nosymbols.
 16. The apparatus of claim 9, further comprising transmittingthe timing related to position based on the first and the secondposition location signals to a position location center.
 17. Acomputer-readable medium including program code, stored thereon, todetermine a position of a mobile device in a time division multiplexedcommunication system, comprising: program code to receive a synchronizedposition location slot, wherein the synchronized position location slotincludes a plurality of sub-slots assigned to a plurality of positionlocation signal sources, and an identifier indicating the synchronizedposition location slot includes position location information; programcode to determine from a first sub-slot in the synchronized positionlocation slot, a first position location signal corresponding to a firstposition location signal source; program code to determine from a secondsub-slot in the synchronized position location slot, a second positionlocation signal corresponding to a second position location signalsource; and program code to determine a timing related to position basedon the first and the second position location signals.
 18. Thecomputer-readable medium of claim 17, wherein the identifier comprises aMAC address reserved for position location.
 19. The computer-readablemedium of claim 17, wherein the first position location signal comprisesa pilot burst from the first position location signal source.
 20. Thecomputer-readable medium of claim 17, wherein the first positionlocation signal comprises a PN sequence corresponding to the firstposition location signal source.
 21. The computer-readable medium ofclaim 17, wherein the first sub-slot is separated in time from thesecond sub-slot by a predetermined guard period.
 22. Thecomputer-readable medium of claim 17, wherein the second sub-slotimmediately follows the first sub-slot.
 23. The computer-readable mediumof claim 17; wherein the second position location signal is receivedduring a period of time in which the first position location signalsource transmits no symbols.
 24. The computer-readable medium of claim17, further comprising: program code to transmit the timing related toposition based on the first and the second position location signals toa position location center.
 25. An apparatus for determining a positionof a mobile device in a time division multiplexed communication system,comprising: An RF frontend for receiving a synchronized positionlocation slot, wherein the synchronized position location slot includesa plurality of sub-slots assigned to a plurality of position locationsignal sources, and an identifier indicating the synchronized positionlocation slot includes position location information; and a basebandprocessor for: determining from a first sub-slot in the synchronizedposition location slot, a first position location signal correspondingto a first position location signal source; determining from a secondsub-slot in the synchronized position location slot, a second positionlocation signal corresponding to a second position location signalsource; and determining a timing related to position based on the firstand the second position location signals.
 26. The apparatus of claim 25,wherein the identifier comprises a MAC address reserved for positionlocation.
 27. The apparatus of claim 25, wherein the first positionlocation signal comprises a pilot burst from the first position locationsignal source.
 28. The apparatus of claim 25, wherein the first positionlocation signal comprises a PN sequence corresponding to the firstposition location signal source.
 29. The apparatus of claim 25, whereinthe first sub-slot is separated in time from the second sub-slot by apredetermined guard period.
 30. The apparatus of claim 25, wherein thesecond sub-slot immediately follows the first sub-slot.
 31. Theapparatus of claim 25, wherein the second position location signal isreceived during a period of time in which the first position locationsignal source transmits no symbols.
 32. The apparatus of claim 25,,further comprising the RF frontend for transmitting the timing relatedto position based on the first and the second position location signalsto a position location center.